Project Summary/Abstract During motor learning, the cerebellum encodes memories of sensorimotor associations that predict deviant action and, during recall of these associations, it will impose adaptive changes to instill corrective behavior. This memory process depends on plasticity that alters the output of the cerebellum through learned patterns of Purkinje cell spike output. Molecular layer interneurons (MLIs) are excited by parallel fibers that convey sensorimotor information relayed through the mossy fiber pathway and, in turn, exert feedforward inhibition onto postsynaptic Purkinje cells to reduce their spike output. MLI synapses are plastic and therefore may be susceptible to learning-induced modification that would alter their inhibitory influence on Purkinje cells and, in this way, impart adaptive behavior. Yet, a basic understanding of how MLIs are affected by experience and if their activity is necessary for the expression of learning is unknown, creating a knowledge gap in the understanding of cerebellar function. Therefore, the objective of this study is to elucidate the role of MLIs in adaptive motor control in behaving mice and measure for learning-induced plasticity in their response properties. This will be accomplished in two aims. In the first, we will use electrophysiology and genetically encoded effectors of activity to measure and manipulate MLI responses in vivo during a motor-learning behavior: adaptation of the vestibulo-ocular reflex (VOR). This will allow us to determine if learning alters how MLIs are activated during sensorimotor stimulation and if their inhibitory output is necessary for pattern changes in Purkinje cell spiking and the expression of learned eye movements. In the second aim, quantitative measurements from cerebellar slice preparations of mice that gave undergone VOR learning will be used to determine if MLIs show activity- induced plasticity in their synaptic properties. This study encompasses an innovative, multidisciplinary approach to decipher the cellular- and circuit-level mechanisms that allow the cerebellum to encode memories of motor learning and implement adaptive motor behavior. Completion of these aims will contribute to novel insights into understanding how the cerebellum stores and recalls memories of learning.